This invention relates to an ultrasonic encapsulation of electronic devices. More particularly, this invention relates to hermetically sealing a moisture sensitive electronic device, such as organic light emitting devices (OLED).
It is known in microelectronic industry that in order to be functionally efficient for their intended purposes, certain types of electronic devices must be sealed to protect them from the ambient environment. Various electronic devices require humidity levels in a range of about 2500 to maximum 5000 parts per million (ppm) to prevent premature degradation of device performance within a specified operating and/or storage life of the device. Control of the environment to this range of humidity levels within a packaged device is typically achieved by encapsulating the device and a desiccant within a cover.
Particular electronic devices, for example, OLED, polymer light-emitting devices, charge-coupled device (CCD) sensors, and micro-electro-mechanical sensors (MEMS) require humidity control to levels below about 1000 ppm and some require humidity control below even 100 ppm. Such low levels of humidity are not achievable using commonly used desiccants like silica gel or Drierite(copyright) materials. Molecular sieve materials can achieve humidity levels below 1000 ppm within an enclosure if dried at a relatively high temperature. However, molecular sieve materials have a relatively low moisture absorption capacity at humidity levels at or below 1000 ppm. Therefore, the minimum achievable humidity level using molecular sieve materials as a desiccant is a function of temperature within an enclosure. Moisture adsorbed by molecular sieve material, for example, at room temperature, can be released into the enclosure or package during temperature cycling to higher temperature, such as, for example, to a temperature of 100xc2x0 C. Desiccants used within such packaged devices include metal oxides, alkaline earth metal oxides, sulfates, metal halides, or perchlorates, i.e., materials having desirably relatively low values of equilibrium minimum humidity and high moisture capacity. However, such materials often chemically absorb moisture relatively slowly compared to the above-mentioned molecular sieve, silica gel, or Drierite(copyright) materials. Such relatively slow reaction of desiccant with water vapor leads to a measurable degree of device degradation in terms of performance.
Numerous publications describe methods and/or materials for controlling humidity levels within enclosed or encapsulated electronic devices. For example, Kawami et al., European Patent Application EP 0776 147 A1 discloses an organic EL element enclosed in an airtight container which contains a desiccant comprised of a solid compound for chemically absorbing moisture. The desiccant is spaced from the organic EL element, and the desiccant is consolidated in a predetermined shape by vacuum vapor deposition, sputtering, or spin coating.
Shores discloses in U.S. Pat. No. 5,304,419 a moisture and particle getter for enclosures which enclose the electronic device. A portion of an inner surface of the enclosure is coated with a pressure sensitive adhesive containing a solid desiccant.
Shores discloses in U.S. Pat. No. 5,401,536 a method of providing a moisture-free enclosure for an electronic device in which the enclosure contains a coating or adhesive with desiccant properties. The coating or adhesive comprises a protonated alumina silicate powder dispersed in a polymer.
Shores discloses in U.S. Pat. No. 5,591,379 a moisture gettering material composition for hermetic devices. The composition is applied as a coating or adhesive on the interior surface of a device packaging, and the composition comprises a water vapor permeable binder, which has dispersed therein a desiccant which is preferably a molecular sieve material.
However, the use of proper desiccant inside the enclosure of an electronic device may extend the service life, but it does not make the device immune to failure due to moisture permeation from the outside ambient. True hermetic sealing of the electronic components can provide a controlled environment in which the electronic devices function, however, a true and effective hermetic seal is difficult to achieve in all the cases. More particularly it is difficult for OLED because of the temperature restraint. The organic layers in the OLED tend to deteriorate if they are exposed to over 100xc2x0 C. for any length of time. In the traditional hermetic sealing, as for example, vacuum tube and lamp manufacturing method, elevated temperatures, high vacuum and other means are employed to accomplish that. The OLED displays do not withstand traditional methods of hermetic sealing as mentioned above.
It is an object of the invention to provide a highly effective hermetically sealed enclosure for moisture sensitive electronic devices.
It is another object of the present invention to provide a method of sealing a cover plate to provide an enclosure using a low temperature metal alloy.
These and other objects are achieved by a method of providing a hermetically sealed enclosure for a highly moisture sensitive electronic device comprising the steps of:
(a) depositing an electrical conductor on the substrate which provides an electrical connection with the electronic device and depositing an electrically insulating thin layer over the electrical conductor;
(b) providing a low melting point indium metal or alloy in the form of a wire having diameter ranging from 1 mil to 10 mils on the electrically insulating thin layer on the substrate so that the wire is situated around the periphery of the electronic device;
(c) placing an ultrasonic horn over the cover plate so that ultrasonic energy can be delivered through the cover plate to the wire and applying a pressure between 20 and 100 psi; and
(d) providing an acoustic wave from the ultrasonic horn through the cover plate to melt the wire.
It is an advantage of the present invention that the hermetically sealed enclosure provided by the above method prevents moisture from permeating through the seal and thus degrading the enclosed electronic device. The present invention is particularly suitable for use with highly moisture sensitive electronic devices like OLED display.